Best graphics card for Blender Cycles rendering
This is hard to say, because there are not relevant benchmark for cycles. (If someone know one, please let us know). 

You can look at http://blenchmark.com/gpu-benchmarks. Keep in mind than this is not accurate results, because results are affected by CPU speed quite enough. 

And yes, you are right: more CUDA cores are better for Cycles. "x80" and "Ti" have also wider bus (384bits) and that is the reason why 780 is little bit faster than 970 (with 256bits) in rendering. But 780 have almost double power consumption. Good for 970 is also 4GB RAM. 

In my opinion the best what you can do is buy one GTX 970 now and second one (or better) later. In Blender you can use multiple CUDA card and can be different.